ADDITIONAL SYSTEM INFORMATION :
JDK 11.0.2

A DESCRIPTION OF THE PROBLEM :
java.net.URL.isOverrideable(String) for "jrt" or "file" returns false,
but custom URLStreamHandlerFactory can override URLStreamHandler for jrt or file protocol
providing the default URLStreamHandlerFactory.

In java.net.URL#getURLStreamHandler,
custom URLStreamHandler's createURLStreamHandler is invoked twice.
But the 2nd invocation does not check by isOverrideable(protocol).

So, custom URLStreamHandlerFactory can override URLStreamHandler for jrt or file protocol
providing the default URLStreamHandlerFactory.

        ==== the 1st invocation ====
        if (isOverrideable(protocol) && jdk.internal.misc.VM.isBooted()) {
            // Use the factory (if any). Volatile read makes
            // URLStreamHandlerFactory appear fully initialized to current thread.
            fac = factory;
            if (fac != null) {
                handler = fac.createURLStreamHandler(protocol);
                checkedWithFactory = true;
            }

            if (handler == null && !protocol.equalsIgnoreCase("jar")) {
                handler = lookupViaProviders(protocol);
            }

            if (handler == null) {
                handler = lookupViaProperty(protocol);
            }
        }

        ==== the 2nd invocation ====
            // Check with factory if another thread set a
            // factory since our last check
            if (!checkedWithFactory && (fac = factory) != null) {
                handler2 = fac.createURLStreamHandler(protocol);
            }

REGRESSION : Last worked in version 11.0.1

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Execute the following test case to reproduce

java CustomURLStreamHandlerFactory jrt:/foo

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
[output]
Default URLStreamHandler used for jrt:/foo
ACTUAL -
[output]
java.lang.RuntimeException: CustomURLStreamHandler used for jrt:/foo
        at CustomURLStreamHandlerFactory$CustomURLStreamHandler.openConnection(CustomURLStreamHandlerFactory.java:13)
        at java.base/java.net.URL.openConnection(URL.java:1051)
        at CustomURLStreamHandlerFactory.main(CustomURLStreamHandlerFactory.java:22)

---------- BEGIN SOURCE ----------
import java.io.*;
import java.net.*;

public class CustomURLStreamHandlerFactory implements URLStreamHandlerFactory {
    @Override
    public URLStreamHandler createURLStreamHandler(String protocol) {
        return new CustomURLStreamHandler();
    }

    private static class CustomURLStreamHandler extends URLStreamHandler {
        @Override
        protected URLConnection openConnection(URL u) throws IOException {
            throw new RuntimeException("CustomURLStreamHandler used for " + u);
        }
    }

    public static void main(String[] args) {
        URL.setURLStreamHandlerFactory(new CustomURLStreamHandlerFactory());

        for (String url : args) {
            try {
                new URL(url).openConnection();
                System.out.println("Default URLStreamHandler used for " + url);
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    }
}
---------- END SOURCE ----------

FREQUENCY : always